The maintenance of quality standards in X-ray photography requires precise control of exposure, source intensity, development procedures, and film characteristics, together with control of environmental variables. Monitoring of these factors is normally accomplished by measuring the density of the developed film by determining its light-transmission characteristics. This is done with an instrument commonly referred to as a densitometer. A beam of light from a standardized light source is caused to pass through a sample of the film, and a detector is positioned to respond to this light. The detector is normally a photo-voltaic device that produces a current output directly proportional to input light intensity over several orders of magnitude. This output is then delivered to a measuring system that converts input current to an output voltage, and a readout expressed as the logarithm of the ratio of the transmitted light through the film to the intensity of the transmitted light without the film being present. Standard procedure has currently involved the establishment of the light intensity without the film, as a matter of calibration. The film sample light transmission is then measured by the instrument, which usually yields a direct reading of the logarithm of the film transmission ratio, referred to as density.
These instruments have been subject to a variety of problems centering in the variation of the intensity of the test light source. Manufacturing variations, and the effects of heating and aging of the bulb components, are inevitably present, together with variations in the line voltage supplying the bulb. The latter are normally compensated for to some extent by either voltage regulation to a nominal level, or by feed back control of the lamp intensity by controlling the voltage on the lamp to eliminate fluctuations. These compensating arrangements have tended to become costly and power consuming, and to be rather ineffective in controlling all of the variables that produce alterations in the lamp brightness.